This invention relates generally to a magnetic ink for nonimpact printing. More particularly, it relates to an ink containing magnetic particles and surface active agents for an ink transfer sheet. Heat and magnetism are used to transfer the ink to a recording medium during printing.
Several attempts have been made to commercialize a small low-priced non-impact printer which prints with magnetic ink. For example, Japanese laid open application No. 96541/1977 discloses one such method wherein ink is transferred by selectively melting a portion of the ink to be recorded. A magnet attracts magnetic particles dispersed in the ink, thereby enhancing the transfer of ink to the recording medium.
An example of prior art magnetically enhanced ink transfer printing is illustrated in FIG. 3. A magnetic ink transfer sheet 12 including a support film 33 and a magnetic ink layer 34 is positioned intermediate a thermal print head 31 and a recording medium 35. A magnet 36 is disposed on an opposed surface of recording medium 35. To print with the illustrated apparatus, thermal print head 31 contacts support film 33 of magnetic ink transfer sheet 12 as magnetic ink layer 34 of ink transfer sheet 12 contacts recording medium 35. Thermal print head 31 selectively applies heat through support film 33 into ink layer 34 to melt specific portions 34' of ink layer 34. Melted portions 34' adhere to recording medium 35 when ink transfer sheet 12 is stripped from recording medium 35, in the direction of arrow A, after printing. A magnet 36 exerts magnetic force on ink 34 to enhance adherance of printed portions of ink 34' to recording medium 35 when ink transfer sheet 12 is peeled away. Thus, this magnetically enhanced printing method generally yields high quality printing of characters and images.
However, printing with the apparatus shown in FIG. 3 is not without shortcomings. During printing, portion of ink to be transferred 34' contacts both support film 33 and the non-recorded ink of ink layer 34 while ink transfer sheet 12 is peeled off. Therefore, the ink intended to be transferred can unintentionally be peeled off with the non-recorded ink because it adheres to non-recorded ink when ink transfer sheet 12 is peeled off recording medium 35.
In ordinary heat transfer printing, complete ink transfer is only possible when the following relationship is established, as illustrated in FIG. 4: EQU FA and FB&gt;&gt;FC and FD
wherein FA is an adhesive force between an ink portion to be transferred 42 and a recording medium 44; FB is a cohesive force of ink to be transferred 42; FC is an adhesive force between ink to be transferred 42 and support film 41; and FD is a cohesive force between ink to be transferred 42 and ink which remains affixed 43 to support film 41. Forces FC and FD act to retain ink to be transferred 42 to support film 41. Forces FA and FB act to transfer ink 42 to recording medium 44.
The transfer efficiency of the apparatus illustrated in FIG. 3 is somewhat increased by magnetic assistance. By magnetically pressing molten ink 34 onto recording medium 35, FA is increased. However, increasing FA does not decrease FC or FD because support film 33, ink 34 and transfer medium 35 are all contacting each other when ink transfer sheet 12 is peeled off recording medium 35. Further, if recording medium 35 has a rough surface, FA will decrease. Because the addition of magnetism does not affect FC or FD, a transfer medium having a rough surface tends to create the situation in which FA is less than FC or FA is less than FD. This could lead to incomplete ink transfer.
Another disadvantage of prior art thermal or magnetically assisted thermal printing is that the dots of ink (pixels) have abnormal shape if a recording medium 52 has a rough surface, as shown in FIG. 5. If recording medium 52 has a rough surface, the surface will have a plurality of recesses 54 and projecting bumps 56. Because contact ink layer 53 does not contact recesses 52, force FA is equal to 0 opposite recesses 54. Frequently, ink will not be transferred to recess 54, but instead, ink will clump at projecting bumps 56. Further, when printing on an extremely rough recording medium (having a Bekk smoothness of about one or two seconds) the magnetic force attracting ink 53 into recording medium 52 is not as effective in sufficiently increasing FA as when printing onto a smooth recording medium. As illustrated in FIG. 6, magnetic ink will only adhere to the surface fibers of a projecting portion 61 to form a recorded dot 62 of irregular shape.
Another defect in conventional magnetic ink printing occurs when magnetic ink layer 34 contacts recording medium 35 during printing. Recording medium 35 acts as a heat sink and absorbs a large portion of the heat generated by thermal print head 31. This phenomenon is referred to as "heat loss". Because of the heat absorbed by recording medium 35, print head 31 must generate considerable heat to melt recorded portions of ink layer 34 to make up for the heat lost to recording medium 35.
By printing with thermal head 31, ink transfer sheet 12 and transfer medium 31 all in contact with each other, friction and heat conduction occurrs between magnetic ink 34 and recording medium 35. This causes heat to build up in magnetic ink layer 34, melting portions of ink layer 34 which were not intended to be transferred. This unintentional transfer of ink is referred to as "greasing".
Conventional magnetic ink includes pigment and fine inorganic magnetic powder dispersed in a binder. The binder frequently includes waxes, thermoplastic resin, dispersing agents, oils, fats and low molecular weight organic substances. Printing with conventional magnetic ink can result in poor transfer efficiency, poor dot reproduceability and irregularly shaped printed dots. Accordingly, the print quality is poor. These defects are intensified when print density is high, i.e., printing a large number of minute dots in a small area.
Accordingly, it is desirable to provide an improved ink composition which overcomes these defficiencies in the prior art.